In short:
Fingerprint sensor broke so phone did not operate reliably.
Tanks to some threads here; I learned how to easily disable it.
(I used thin insulation tape on the sensor contact pads, reachable after removing main board.)
(Search forum for fingerprint sensor)
Longer history:
I dropped my fp3 and unfortunately it hit a sharp stone with the fingerprint sensor.
Result: phone froze, and reader got very hot.
Battery out & in, charger connect. Sensor hot but not as much.
Phone booted, after some attempts, very sluggish, and fingerprint reader do not work.
By some hours sensor was less and less hot, and phone usable, but sometimes sluggish and sometimes failed booting. Sensor never works.
I guess whatever got short circuited burnt off, or he supply to it broke down nicely.
And the system tried in vane to communicate with the sensor but did not handle bad communication well.
I read in support that sensor is part of main module and that cost too much to replace.
I needed a bit better camera and performance anyway so i bought a fp5 (with murena /e/os) and decided to try to fix my fp3 later to be my new spare phone.
Now, I finally searched this forum (should have done that to begin with! )
And your information here helped me fix it in ten minutes!
So thank you forum members for the threads, and the organisation for he forum! 